Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving surface to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 dpi and 600 dpi.
Ink-jet printing systems commonly utilize either a direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving web. In an offset printing system, the image is formed on an intermediate transfer surface and subsequently transferred to the final receiving web. The intermediate transfer surface may take the form of a liquid layer that is applied to a support surface, such as a drum. The print head jets the ink onto the intermediate transfer surface to form an ink image thereon. Once the ink image has been fully deposited, the final receiving web is then brought into contact with the intermediate transfer surface and the ink image is transferred to the final receiving web.
FIG. 1 provides a simplified view of a direct-to-sheet, continuous-media, phase-change ink printing machine. A media supply and handling system is configured to supply a long (i.e., substantially continuous) web of media W of “substrate” (paper, plastic, or other printable material) from a media source, such as spool of media 10. In certain printing machines the web W passes through a series of tensioning rollers 12 to a pre-heater 18 that brings the web to an initial predetermined temperature that is selected for desired image characteristics corresponding to the type of media being printed as well as the type, colors, and number of inks being used. The media is then transported through a printing station 20 that includes a series of print head modules 21A, 21B, 21C, and 21D, each printhead module effectively extending across the width of the media and being able to place ink directly (i.e., without use of an intermediate or offset member) onto the moving media. As is generally familiar, each of the print heads may eject a single color of ink, one for each of the colors typically used in color printing, namely, cyan, magenta, yellow, and black (CMYK). Image data obtained from an image processor, such as a scanner (not shown) is provided to a controller 22 that controls the operation of the print heads as well as the delivery of molten ink from the ink supply 24 to the print heads.
Following the printing zone 20 along the media path are one or more “mid-heaters” 30 that may use contact, radiant, conductive, and/or convective heat to control a temperature of the media. The mid-heater 30 brings the ink placed on the media to a temperature suitable for desired properties when the ink on the media is sent through the fixing assembly 40. The fixing assembly 40 is configured to apply heat and/or pressure to the media to fix the images to the media. The fixing assembly may include any suitable device or apparatus for fixing images to the media such as an image-side roller 42 and a pressure roller 44, both configured to apply heat and pressure to the media. Nip rollers 50 are provided at the outlet of the fixing assembly to guide the substrate to a receiving station (not shown).
The printing machine may use “phase-change ink,” by which is meant that the ink is substantially solid at room temperature and substantially liquid when heated to a phase change ink melting temperature for jetting onto the imaging receiving surface. The phase change ink melting temperature may be at any temperature that is capable of melting solid phase change ink into liquid or molten form. In certain printing machines, the phase change ink melting temperature is approximately 70° C. to 140° C. The molten ink supply 24 for a phase-change ink system thus includes a melting station having a melter 25 that melts solid ink elements received from a hopper 26. In certain embodiments the solid ink elements are in the form of pellets that are fed from a solid ink supply 27 through a feed conduit 28 to the hopper. The supply 27 is replenishable, meaning that it can be re-filled with solid ink pellets or replaced with a fully loaded supply container.
High usage or throughput printing systems typically require large solid ink supplies 27 that do not require frequent replenishment. Thus, in such high throughput systems the supply is in the form of one or more large drums, such as a 55 gallon drum. A solid ink supply of this magnitude can accommodate high ink usage rates (on the order of 33 gallons per color per day) without placing an undue burden on the operator to constantly replace or replenish the solid ink supply.